1. Field of the Invention
The present invention relates generally to a method of dressing a grindstone used in an NC (numerically controlled) grinding machine, and more particularly to a dressing method allowing a rotary dresser to cut into the grindstone at a precise position.
2. Description of the Related Arts
A typical NC grinding machine includes a grindstone formed from ultra-abrasive grains such as diamond or CBN (cubic boron nitride). The grindstone having the ultra-abrasive grains has two or three times the hardness of a conventional one formed from general-abrasive grains such as aluminum oxide (alumina) or silicon carbide, and is resistant to abrasion and fragmentation, and is therefore suitable for efficiently grinding a workpiece with close dimensional tolerances. Also, due to the high abrasion resistance of the ultra-abrasive grains, a binder is allowed to have a higher strength, with the result that the diameter of the grindstone hardly varies and hence machining dimensions of the workpiece can be stabilized.
Since the grindstone having the ultra-abrasive grains is more expensive compared with the general grindstone, it is preferable to effectively dress the former with a minimum dressing amount. For this reason, in the case of dressing the grindstone having the ultra-abrasive grains in the NC grinding machine, a rotary dresser must cut into the grindstone at a precise position.
However, a distance between the rotary dresser and the grindstone during dressing may vary depending on a thermal expansion of the NC grinding machine caused by heat generated during grinding, or on a thermal shrinkage thereof caused by a change in the ambient temperature. If the distance is short, the depth of cut made by the rotary dresser may become too large, whereas if the distance is long, it is possible that no dressing may be performed since the rotary dresser does not come into contact with the grindstone. In the case where the dressing fails because of non-contact, for example, in an automated production line, the products may have poor surface finishes, leading to damage along the production line.
Thus, in the prior art, to ensure that the rotary dresser cuts into the grindstone at a precise position, contact of the rotary dresser with the grindstone is detected, and the detected position of the point of contact is used to correct an NC command value for specifying a position of the rotary dresser, issued from a numerical controller. A vibration sensor is mounted on the rotary dresser unit for detecting the contact of the rotating rotary dresser with the grindstone.
In a method of correcting the NC command value by means of the vibration sensor in this manner, if a failure occurs in the vibration sensor, or there is a break or defective contact in connection cables, the rotary dresser may continue to advance toward the grindstone, thereby damaging the grindstone or the rotary dresser itself. Further, if a droplet of, for example, the coolant touches a detection part of the vibration sensor, a contact signal may be generated irrespective of actual non-contact of the rotary dresser with the grindstone, resulting in an insufficient dressing operation.